The present invention relates to a video data processing device. More particularly, the present invention relates to a video data display device suitable for displaying images or character information contained in video signals on a display device for computers such as a personal computer and a work station.
In recent years, the use of multimedia techniques has risen. Thus there has been an increased need for superimposing images of video signals for television, etc., for displaying images on a graphic display screen for computers such as a personal computer (hereinafter, abbreviated to "PC"), or a work station (hereinafter, abbreviated to "WS"). Image display devices for realizing this need have also been developed and are now available.
Further in recent years, a PC obtained by incorporating a television tuner in addition to a mechanism of superimposing images of video signals on a graphic display screen has been available as a "personal computer in which television can be seen".
There exist plenty of systems of superimposing images thus contained in video signals on a graphic screen for computers.
Buses (for example, PCI bus) having fast transfer velocities capable of transferring tremendous digital image data on real-time basis are now available and widely used with PC's or WS's. As a result there has been developed a system (hereinafter, referred to as "direct transfer system") for displaying images by transferring digitized video signals to a memory for graphic display for PC's or WS through such buses.
With this direct transfer system, an image can be displayed only by transferring data through bus once and therefore, it is possible to transfer a large quantity of image data within a unit time, and to display high-quality images.
In this respect, for details of this direct transfer system, refer to the April 1996 issue of Interface Journal (on pages 102 to 109) published by CQ Publishing Company.
Also, in Japanese Patent Laid-Open Application No. 6-124189, there is disclosed a technique of superimposing images in video signals on a graphic screen for displaying by using the direct transfer system.
On the other hand, some video signals contain character information in addition to images. For example, Closed Caption (hereinafter, abbreviated to "CC") standardized by FCC (Federal Communications Commission) in the United States is a system of multiplexing character information such as captions in addition to images.
In the United States, is multiplexed with video signals, character information related to screen using the CC system for persons who have difficulty in hearing. According to US laws, from Apr. 1, 1993, the obliged all television sets of 13 inch or more sold in the United States must have the ability to respond to the CC system.
With the CC system, character information is multiplexed to the area of a period which is not displayed in a screen called a "vertical blanking interval" among video signals. For this reason, character information multiplexed by the CC system is not displayed on the screen in television sets, etc. having no decoder of the CC system, but when a television set in which a decoder of the CC system is incorporated receives video signals with character information multiplexed, the captions can be seen together with images.
Currently, there are television sets etc., on the market, which are capable of displaying, on the screen, characters obtained by decoding character information multiplexed to video signals by using the CC system. There are also a decoding device or VTR (Video Tape Recorder), on the market, having a function to multiplex character information for outputting, which is multiplexed to video signals by the CC system, to images likewise contained in the video signals.
The technique related to the above described in the Interface Journal or the technique disclosed in the Japanese Patent Laid-Open Application No.6-124189 had the problem that no consideration has been paid to character information contained in video signals.
This is because the appropriate treating methods in PC are different between video data such as images and data of character information.
Generally, video data such as images have a large amount of data, and a load is applied to the system in the mode of execution in which CPU of computer interprets software, and therefore, exclusive hardware such as image board of MPEG board, etc. is mostly prepared. Also, when video data are transferred to CPU, there is also the problem that the traffic of bus increases to lower the performance of the entire system.
On the other hand, the character information has smaller amount of data than the video data, and is suitable for the mode of execution in which CPU interprets software. Processing of this character information by the use of exclusive hardware has the problem that the device will be larger in size and expensive. Also, preparation of the respective exclusive hardware every time character information is processed has the problem that it lacks flexibility. Further, the use of the exclusive hardware has the problem that the power consumption will be high.
When images and characters are thus superimposed on a graphic display screen, there is the problem that the processes for video data and character information data must be properly divided into a process by the exclusive hardware and a process in which a CPU interprets the software for execution.